Refractory metal bodies and method of making same



United States Patent REFRACTORY METAL BODIES AND METHOD OF MAKING SAME No Drawing. Application January 15, 1953 Serial No. 331,498

Claims. (Cl. 29-198) This invention relates to refractory metal bodies and to other metal bodies which have been clad with a refractory metal and coated with a sintered skin containing silicon and zirconium, with or without further quantities of boron to modify the effect thereof. These sintered skins serve to impart to the refractory metal body a high resistance to oxidation in air at high temperatures. More particularly, the invention relates to molybdenum or materials which have been clad with molybdenum to which substantial oxidation resistance in air at high temperatures has been imparted by a skin coating thereonof silicon and zirconium, with or without further additions of boron, said skin being an alloy or intermetallic composition with the molybdenum, and to mechanical methods of forming these metallic products.

Refractory metals, particularly molybdenum, have highly desirable mechanical properties at elevated temperatures. One desirable use therefor has been as electrical furnace heating elements. Other desirable uses are in oil burner nozzles, artillery piece nozzles, rocket nozzles, turbine'blades and buckets, component parts of jet engines, ignition coils for burners and valve seats for internal combustion engines. To obtain optimum utility for refractory metals in these several high temperature I uses it is usually necessary to exclude oxygen, and it is from oxidation. The coating therein is an integral coat- ,ing or skin upon the molybdenum base, essentially of MoSi as alloys or intermetallic compositions having a silicon to refractory metal content in the molecular ratio of from about 1:1 to about 3:1, which correspond to alloys or intermetallic compositions containing from about 22.5% to about 47% silicon. Although the optimum protection of the molybdenum is obtained with coatings or skins having a molecular ratio of silicon to molybdenum of about 2:1, corresponding to a silicon content of about 37%, coatings or skin beyond this composition range also aiford some protection for the molybdenum base or core. Thus, where it is indicated 2,870,527 Patented Jan. 27, 1959 "ice that the exterior skin layer comprises about 37% silicon and the balance molybdenum, this does not mean that the exterior layer consists entirely of the intermetallic compound of silicon and molybdenum having the approximate percentage named, namely McSi but that the exterior layer consists largely or essentially of that pure intermetallic compound and may have associated with it the intermetallic compound of molybdenum and silicon consisting of about 22% silicon and the balance molybdenum known as MoSi, and/or silicon in an amount in excess of that required to form the compound Mosiz. I

These alloy coatings or skins on molybdenumfurnish an exceedingly high resistance to oxidation at elevated temperatures. For example, molybdenum wire of very fine diameter of approximately 20 mils has a life of approximately 16 seconds when heated to a temperature of about 1500" C. in air, The same size molybdenum wire when provided with a coating or skin as above de-. scribed, having a thickness of about 0.32 mil, has a life of 4000 seconds in air at the same temperature.

This thickness of coating does not represent the total thickness of an alloy layer. The total thickness of the alloy layer is roughly about double the thickness increase which is obtained during the coating operation.

Throughout this application the term thickness is i September 26, 1952, now Patent No. 2,804,406 (Yntema et al.), improved resistance of molybdenum to high temperature oxidation in air was found by mechanically applying a mixture of silicon and boron wherein the mechanically applied coating, in a ratio of about 1 to 4 parts of boron to 9 to 6 parts of silicon, is sintered to the molybdenum as a skin alloy or intermetallic composition of molybdenum silicon and boron.

Molybdenum wire may fail at the point where it is maintained at highest temperature. molybdenum wire is secured between water cooled electrodes with passage of an electrical current therethrough,

it may fail at its hottest pointabout the center of the wire. A second type of failure is found at a point adjacent the cooler juncture of the wire with an electrode.

Still another type of failure is at an intermediate point,

neither the hottest nor the coolest.

It is apparent that the life of molybdenum having a protective coating or skin is not necessarily and solely dependent upon the maximum temperature to which the body may be subjected. .Molybdenum-silicon-boron coatings are improved over molybdenum-silicon coatings in that the tendency of the wire to fail at a point of intermediate temperature is substantially reduced. However, such molybdenum-silicon-boron coatings, while having a long 1ifeup to about 250 hours--at about 1700 C. in air, rapidly fail at a higher temperature.

For example, where 3v in. all of the coatin s referred to herein, the, increased thickness of the coated molybdenum base does not represent the total thickness of the coating per se inasmuch as 1n the hot application, either sintering or by vapor deposition to effect the coating, there is substantial penetration of the molybdenum base. However, the thickness of coating as referred to herein is measured as a thickness increase from the original thickness of the metal coated and in the actual thickness of the alloy or intermetallic composition imparted as a skin to the molybdenum. Such thickness of coating usually exceeds about ;5 mil' and may range-from about 1 to 5 mils, the. thicker coatings usually having the greater protection. According to the. present invention, it is found that a coating of zirconium together with silicon, applied to the molybdenum, as an alloy or intermetallic composition therewith, will impart-thereto such resistance to oxida tlon. in air that it will withstand a temperature well ex- Ceeding 1700 C., such as above even about 2000 C., for Substantial periods, at which temperatures, and even at substantially lower temperatures, all coatings such as silicon and combinations thereof with boron, as taught in the applications above referred to, will have failed. In contrast, while a coating of molybdenum with silicon and boron is stable at 1700 C. in air for about 250 hours, the boron immediately begins to bubble and the coating is quickly destroyed by flakingand blistering at a tempera ture in the range of 1800 to 2000 C. However, in accordance with the present invention, when boron and zirconium are used with silicon, the coating will not only withstand temperatures up to 1700 C. for a longer time, but the. boron in combination therewith tends to inhibit the flaking and decrease a tendency of silicon and zirconiurn alone, to fail in a low temperature zone of heat, while increasing the life of the, zirconium-silicon coated molybdenum over the entire range of temperatures.

For example, silicon alone, while it will protect molybdenum for about 35 hours at 1700 C. in air, will protect it only'about 1 hour at 2000 C. and will fuse at 2100" C. In contrast, silicon and boron both coated upon molybdenumwill extend the life at 1700 C. about 8 times, but will provide a useful life in air at 2000 C. for only about 3 hours. Silicon and zirconium, however, at 2000 C. will give a useful life of about 9 hours, i. e., 3 times as etfective as silicon and boron and 9 times as efiective as silicon alone to stabilize the molybdenum against oxidation in air at the high temperature of 2000 C. Moreover, this coating will not fuse at temperatures above2100 C. The zirconium and silicon coated molybdenum, while increasing the life at this high temperature of 2000 C., is quitebrittle and the zirconium tends to flake, 01f. Accordingly, additional quantities of boron are desirably included in the coating. Surprisingly, while boron-silicon alone will blister and flake off at temperatures of the order of 2000" C.,' giving a life of only about 3 hours, the boron combined with zirconium and silicon, while stabilizing the zirconium-silicon to prevent flaking thereof, does not shorten the life thereof, but gives a coating of about the same life as zirconium-silicon, i. e., about 9 hours at 2000 C., but which is less brittle and nonflaking.

Several procedures are available for applying the coatings hereof to the molybdenum. One useful procedure is to first electroplate or metalize the zirconium upon the molybdenum and then deposit the silicon, or boron and silicon, by contacting the zirconium coated molybdenum -to the method pointed out therein, the zirconium coated molybdenum is contacted with the halide vapors mixed with dry hydrogen at temperatures of 1400 to 1800 C. Where the 'coati'ng'is to contain no boron, then halides of silicon only may be applied as vapors, together with hydrogen, as described in copending applications (Campi i I bell et al.) Serial No. 150,543, Serial No. 150,544 and Serial No. 150,398, filed March 18, 1950, to deposit the silicon upon the zirconium coated molybdenum.

Another desirable method, which is generally preferred because of its simplicity and economy, is to mechanically apply to the molybdenum the elements silicon and zirconium with or without boron, as a slurry in a carrier liquid, together with a temporary binder substance to be painted and dried as a temporary coating upon the molybdenum, which is finally heated to volatilize and destroy the temporary hinder, the elements being ultimately heated to sinter the same into the molybdenum as an alloy or intermetallic composition therewith.

Other useful procedures are available in combinations 4 1 of the procedures set forth above. For example, the molybdenum may be first metalized or plated with zirconium metal and the silicon, with orwithout boron,

may be further applied thereto either mechanically by painting as a slurry thereon or by a temporary binder I which is finally sintered into the molybdenum coated with zirconium; or the silison, with or without boron, may be coated by vapor depositions. Thus, any combination of plating or mechanically applying one or more of the elements by painting and sintering, or deposition of one or more elements from a vapor, may be applied in anydesired sequence to effect the coating.

Still other procedures are possible, particularly in ctfecting the zirconium coating on the molybdenum. For example, the molybdenum may be prealloyed with the zirconium to homogeneously contain a desired quantity In any of these procedures, each of the elements may i V be coated as a separate step, or two of the elements may be first coated upon the molybdenum, with the third coated as a separate step.

silicon with the zirconium, or the silicon with the boron, or the boron with the zirconium; or all three of these elements may be first prealloyed by sintering and then grinding to a powder, which is then mechanically applied by painting with a temporary binder upon the molybdenum, which is finally set as an integral skin upon the molybdenum either as an alloy or intermetallic compo- Ef sition therewith by sintering.

Substantial penetration and interaction of the coating elements takes place with the molybdenum, variable somewhat with the time and the degree of heating as well as the method of application. The silicon with respect to the elements zirconium and boron will vary from approximately 50 to 90% by weight of a mixture therewith. The zirconium will vary from about 25. to 5% by weight of the mixture and the boron, when used, will be in a generally similar proportion to that of the zirconium, usually 25 to 5% by weight. zirconium and silicon alone are applied, the proportions of these elements as coated may be 75 to 95% silicon i to 25 to 5% zirconium, but are preferably in the range of 10 to 20% of zirconium to 90 to of silicon; the ratio of boron, when used, is the same as the zirconium so that in a ternary mixture applied as a coating, the

ratio may range from 50 to silicon to 25 to 5.%

zirconium to 25 to 5% boron, but a ratio of 60 to 30% silicon to 20 to 10% zirconium to 20 to 10% boronis preferred.

The actual composition of the skin formed upon sin- 5 It is ofttimes useful when" mechanical coating procedures are used to prealloy the Usually when tering with the molybdenum is one in which the quantity of molybdenum will vary from 50 to about 75%, increasing in molybdenum from the lowest to the highest in the region wherein the modybdenum is penetrated, the quantity of molybdenum varying progressively to approach the pure molybdenum body. It is believed that the predominant molybdenum compound in the skin is molybdenum disilicide, Mosi but compounds of zirconium and boron will be present. Thus, the skin may also contain such compounds as zirconium disilicide, zirconium borosilicide, molybdenum borosilicide, molybdenum boride, zirconium boride, and silicon boride, and complex combinations of these Whose identities have not been determined, as well as these several elements, molybdenum, silicon or zirconium as alloys.

The useful ratios of the coating elements are set forth in table form:

Considered as an intermetallic composition or alloy with the molybdenum, where zirconium and silicon alone are used the preferred composition for optimum stabilization effect is about 59% molybdenum, 37% silicon and 4% zirconium. Where the boron is also included as a component with the molybdenum, the preferred composition is approximately 59% molybdenum, 33% silicon, 4% zirconium and 4% boron.

In the several procedures mentioned above, where the molybdenum is first coated with zirconium to effect a plating, the zirconized molybdenum is subjected thereafter to vapors of silicon tetrachloride or other volatilizable silicon halide at temperatures in the range of about 1400 to 1800* C. in a reducing atmosphere, such as by volatilizing the silicon halide in an atmosphere of hydrogen. The zirconized and the siliconized molybdenum body may thereafter be again treated by a similar vapor contact comprising -a volatile halide of boron mixed with hydrogen. The vapor contact may be a mixture of vapors of both silicon halide and boron halide in hydrogen, as described in the aforesaid application Serial No. 299,216 (Yntema et al.). A desirable means for heating the molybdenum, after being coated with zirconium, is by electric resistance or by suspending the zirconized molybdenum metal such as a wire between electrodes and passing an electric current therethrough while exposing the same to the vapors of the silicon and boron compounds to be coated thereupon, the temperature of the wire being'regulated by the quantity of current passed therethrough.

. A much simpler and therefore preferred procedure for coating the molybdenum is to apply thereto the metal powderssilicon, zirconium, or additionally, boronas a mixture of these elements, or in sequence in finely powdered form, as a slurry in a liquid carrier comprising a solution of a temporary binder substance which may be subsequently removed by heating to effect evaporation and/or decomposition without leaving a carbonaceous residue in any significant quantity. The slurry is painted, such as by hand brushing, dipping, or spraying upon the molybdenum base. The wet coated molybdenum is then dried at ordinary or raised temperatures, such as in an oven at a temperature, which merely evaporates the solvent, the temperature usually not exceeding about 100 C. The dried coated product is then slowly heated in a non-oxidizing atmosphere, such as hydrogen, over a period of about 1 to 5 minutes to a temperature in the range of about 1300 to 1800 0., usually about 1400 to 1600 C., whereby the temporary binder substance is volatilized and/or decomposed to leave substantially no carbonaceous residue. The coated molybdenum wire is then maintained for a further period of about 1 t0 4 minutes, usually about 2 or 3 minutes, which is sufiicient to sinter the particles into the molybdenum as an integral skin thereon, comprising an alloy or intermetallic composition with the molybdenum.

According to the preferred mechanical application procedure, both silicon and zirconium are applied, preferably in proportions of to silicon to 20 to 10% zirconium. Where the three elements are applied, the preferred range is 60 to 80% silicon to 20 to 10% zirconium to 20 to 10% boron. Powders of these elements separately, or as mixtures or as presintered alloys, are converted to a paint by suspending the finely powdered particles in a liquid paint composition comprising a thermoplastic resin, preferably an alkyd resin such as Glyptal,

dissolved in a solvent. Coatings of the powders upon the molybdenum are mechanically applied by painting a slurry of the elements silicon and zirconium as a binary mixture, or further with boron as a ternary mixture, upon the molybdenum base metal or upon a molybdenumclad base metal such as steel, for example. The elements are temporarily bonded by the carrier to the molybdenum by drying the wet coatings, and are subsequently sintered to an alloy or intermetallic composition of the applied elements as a skin upon the molybdenum to form an alloy or intermetallic composition therewith. The molybdenum may be first coated with a liquid slurry of oneelemcnt, dried and sintered, then coated with a liquid slurry of the other element, and dried and sintered. Alternatively, the molybdenum is coated with a slurry of mixed finely powdered elements of silicon and zirconium, or additionally with boron, which are then dried and sintered. In a further alternative procedure, the silicon and zirconium, or silicon, zirconium and boron, may be sintered into an alloy or fused mixture, and the finely powdered sintered mixture is suspended as a liquid slurry and painted upon the molybdenum, The coating is then dried and finally sintered to the molybdenum. All of these procedures are possible, but the sintering of the finely powdered mixture of elements after application to the molybdenum and temporarily bonded thereto by drying is preferred.

In each case the coating procedure is repeated to produce a sintered intermetallic composition or alloy skin upon-the molybdenum exceeding about 0.5 mil in thickness, such as in the range of l to 5 mils in thickness, preferably about 1.5 to 3 mils. The desired thickness is obtained by successive applications of painted coatings, such as about 2 to 5 successive painted and dried coatings, with or without intermediate sinterings. preferred to sinter each coating until the desired thickness of sintered coating is obtained. There is some variation in the life of the coating with respect to oxidation resistance in air at high temperatures, depending upon the method of sintering, the fineness of the elemental powders applied as a slurry in the wet coating to the molybdenum, and the character of the carrier liquid. Obviously, it is desirable in any case that the coating be uniformly applied in even thickness over the molybdenum to obtain a uniformly thick sintered coating.

The paint resin The resin of this paint is selected to be a thermoplastic resin because of its superior property to be volatilized and/or decomposed at high temperatures upon sintering 7 suitable.

of which is magnesia.

m -leave. nosubstantial carbonaceous residue, and for inipartingv fluidity, smoothness and even thickness to this paint. 'Anytypical thermoplastic paint resin which leaves no substantial carbonaceous residue when heated to temperatures. far exceeding its decomposition temperature, such as abovel300 C., may be used herein. Such resin will'be understood to be a typical temporary bonding resin to firmly adhere the powdered elements silicon and zirconium, or additionally boron, to the molybdenum and maintain the adhesion until the resin is completely volat ilized and/ or decomposed. Alkyd resins are superior in this respect since they may be applied as a smooth coating and leave no carbonaceous residue when heated. Thermosetting resins such as Bakelite are generally unaction of polybasic organic acids or their anhydrides, such as phthalic acid, succinic acid, adipic acid or their anhydrides, etc., with a polyhydroxy aliphatic alcohol such as glycerin, ethylene glycol, etc., of which the re action product of phthalic anhydride with glycerin, i. e., Glyptal resin, is preferred. The resin is applied in proportions of from to usually about 10%, by weight of the liquid carrier.

The solvent Desirable, solyents, particularlyfor alkyd resins, are ketones. Thus, we may use acetone, methyl ethyl ketone, diethyl ketone, diacetone alcohol and, preferably for a Qlyptal, a mixture of diacetone alcohol and acetone in a ratio of about 7 :3 by volume may be used.

Zirconium Technically pure elemental zirconium is ground to particle size generally less than 325 mesh, or even finer sized zirconium is desirably used. Finer particles may be obtained by elut-riating a finely powdered suspension of approximately 325 mesh zirconium powder in water by agirating the powder suspension and pipetting ofi portions of the suspension near the surface of the agitated liquid. The zirconium powder thus obtained is considerably finer .than 325 mesh and gives improved coatings when suspended as a slurry in the paint carrier alone or together with the other powdered elements, silicon and boron, as a paint composition which is ultimately sintered upon the molybdenum, as described.

Silicon The silicon is preferably fine commercial elemental silicon, usually about 97% pure, used as a very finely powdered fraction which will pass a 325 mesh sieve or even finer. A desirable form of silicon is obtained by further classifying 325 mesh silicon by stirring a slurry thereof in waterand pipetting off successive portions near the upper surface to. obtain an extremely fine elutriated silicon in this manner.

Boron The boron used may be a commercial grade comprising about 91% of elemental boron, the major impurity The boron is used as a finely powdered product, all particles of which are screened to pass a 325 mesh screen or even finer.

Prcsintering and mixing As indicated above, the zirconium and the silicon or the ternary elements zirconium-silicon-boron may be ap in the resinous solution in the solvent, as a paint upon the molybdenum. Superior results are obtained when thcele'ments'are applied in extremely fine particle size,

Typical alkyds are such as are formed byrcsay lesshan abou 5 m s e r bly, h si on i the extremely fine elutriated fraction obtained as above. described, wherein the particle size is considerably less- Improved results, however, are obtainable where the fine powders, homogeneously mixeddry, are presintered at a temperature ranging from 1300. to. 1800 C. in an inert atmosphere, such as hydrogemandy then finely ground to a powder less than 325 mesh, whiclr' is-suspended as a slurry in the carrier liquid as a coating;

than 325 mesh.

composition.

Coating composition The resin is first dissolved in a high boiling solvent, as I above identified, preferably Glyptal resin in a solvent 7 comprising a 7:3 volumetric mixture of diacetone alcohol and acetone. The. powders are then stirred into the Glyptal resin in the ketone solvent in the desired ratio to I form a smooth paint slurry. The molybdenum or molybdenum clad metal as wire, rod, fiat sheet, or irregular shapes, is then coated therewith by hand brushing, The 1 wet coated metal is then placed in an air drying oven and dried at moderate temperatures, including air temperature, usually not exceeding C. to evaporate the solvent dipping or spraying with an air brush type sprayer.

and produce an empirically dry adherent coating of,

powder temporarily bonded to the molybdenum with the 1 alkyd resin.

.. Sintering The coated molybdenum has its coating sintered by heating for a short period of time in an inert or reducing 7 atmosphere, such as hydrogen, in the temperature range of about l300 to 1800 C., whereby the Glyptal resin binder is volatilized and/or decomposed, leaving substantially no residue of carbon; simultaneously, when heated in this range, the zirconium and silicon, or additionally boron when present, become sintered to a substantially penetrated and sintered alloy or intermetallic I i composition skin upon the molybdenum.

Several types of furnaces may be used for efiecting the sintering. Heating may be in a mufile furnace or an induction heated furnace, or the molybdenum metal base may form the resistance of an electrical circuit whereby it becomes heated to the desired temperature range, herein termed electrical resistance heating. If desired, the dried coated molybdenum may be preheated to more firmly set and partially or entirely decompose or volatilize v the binder. In any case, the coated molybdenum is heated relatively slowly over a period of 1 to 4 minutes to obtain the optimum sintering temperature in the range of' 1300 to 1800 C., preferably about 1500? C. For sin tering, after reaching the sintering temperature, the coated molybdenum is maintained at the sintering temperature for about 1 to 4 minutes, preferably about 2 or 3 minutes.

It will be understood that substantial variation in the time of heating the molybdenum or molybdenum-clad metal is possible, depending upon the size and the rgularity, i. e., shape of the metallic article being heated, the

objective being to relatively slowly heat the article at. a rate which is not so rapid that the temporarily bonded and incompletely set coating will blister or flake off.

EXAMPLE 1 Molybdenum wire having a thickness of approximately 20 mils is plated with zirconium by conventional procedures to a plate thickness of approximately 0.5 mil. The wire is then suspended between electrodes within a chamber which is first purged of air by passing hydrogen gas therethrough and through which is then passed vapors of silicon tetrachloride and hydrogen which has been freed of water vapor and oxygen, the wire being heated by the passage of electrical current therethrough to a temperature of approximately 1600 C. The atmosphere of silicon tetrachloride and hydrogen may be produced by passing the hydrogen gas through liquid silicon tetrachloride .rueint insd at. a e a c of about at a r te of about 800 cc. of hydrogen gas per minute and passed into contact with the hot zirconium coated molybdenum wire over a period of about 8 minutes, and then terminating the heating of the filament while continuing to pass the hydrogen gas alone through the chamber while the filament is cooling. Where boron is desired to be included in the coating, the same method is followed except that both boron and silicon halide vapors are obtained by bubbling hydrogen gas separately through the silicon and boron compounds at separate rates to obtain the desired proportions, after which both vapors and hydrogen are combined before passing into the chamber for contact with the hot filament. Further control of proportions of halide vaporized into the hydrogen is obtained by varying the temperature of the liquid halide of silicon or boron, such as between and 30 C.

, EXAMPLE 2 The following table illustrates a substantial number of comparative coatings, mechanically applied, using silicon alone as a coating, silicon-boron as a coating, siliconzirconium as a coating, as well as silicon-zirconiumboron as a coating. Each of these coatings were obtained by applying a slurry of the element or mixture of elements in a 10% solution of Glyptal resin using diacetone alcohol and acetone in the ratio of 7:3 by volume as a solvent. These coatings were upon 80 mil molybdenum rod, applied thereto by hand-brushing, then drying in an oven at about 100 C., further slowly heating the dry coating over a period of about 4 minutes in an atmosphere of hydrogen to reach a temperature of 1500 C., then maintaining the coated rod at about 1500 C. for about 3 minutes to sinter the coating. The coating operation was repeated in each case until a sintered coating of approximately 2.5 mils in thickness was obtained.

MAXIMUM LIFE IN HOURS OF SAMPLES OF 80- MIL MOLYBDENUM ROD COATED WITH VARI- OUS METALS AND HEATED IN AIR Weight Testing Temperature, C. Coating Metals Percent Ratio 35.2 1.3 Fused.

80:20 252. 3 61. 0 3. 4 Do. 80:20 533.0 2.0 0. 1 Do. 80:20 160. 9 0. 3 Do. 90:10 9. 2 80:10:10 9. 1

l A coating of Al or C was applied on top of the 81-13 coating.

It will be noted that each of the coated rods was tested by heating in air over a range of temperatures. All of the wires were destroyed in a few hours at 2000 C. except the zirconium-silicon coating and the zirconiumsilicon-boron coating, each of which lasted approximately the same time, about 9 hours. Moreover, all of the coatings fused immediately at 2100 C. except zirconium containing coatings, which did not fuse at all. Moreover, the boron-containing zirconium-silicon coating was a smoother, more even coat which showed no tendency to blister and flake, and which was therein superior to the siliconzirconium coating containing no boronr In further contrast, it is pointed out that molybdenum coated with zirconium alone has no effective oxidation resistance.

EXAMPLE 3 The effect of zirconium coatings alone and in combination with silicon and with both silicon and boron are further illustrated. Several coatings were made as described in Example 2, using various combinations of these elements in varying proportions. The coating thicknesses varied from 1.5 to 35 mils. Again, as illustrated, the better proportions are 90:10 where silicon and zirconium alone are used, but fairly good results are obtained in ratios of 80:20, and also good results are obtained with all three elements in ratios of 80:10: 10.

. 10 FABRICATION AND TESTING OF THE OXIDATION RESISTANCE OF Si-B-Zr COATINGS ON -MIL MOLYBDENUM' ROD Ratio of Metals Ap plied, Percent by Sintering Coating Testing Weight Temper- Thickness, Temp., Llfe,

ature, C. Mils 0. Hours Si Zr B l0 1, 500 3. 0 1, 900 N11 10 l, 700 3. 0 1, 900 N11 5 5 1,500 1. 5 1, 900 1. 9 I 5 5 1, 700 1. 5 1, 900 1 5 4 4 2 1, 500 Not sintered 4 4 2 1, 800 N01; sintered 4 4 2 1, 900 Fused s 2 1, 500 3. o 1, 900 5. 4 8 2 1- 1, 500 2. 5 1, 900 9. 9 8 2 1,700 2.0 1, 900 11. 9 10 1,500 2.0 Recoated 10 1, 500 2. 5 2, 000 0.8 7 3 1, 500 3. 0 2, 000 1. 4 7 3 1, 500 2. 5 2, 000 5. 7 7 3 1, 500 2. 5 2, 000 6. 9 s 2 i 1, 500 2. 5 2, 000 4. 7 8 2 1, 500 2. 5 2, 000 2. 1 8 2 1, 600 3. 0 2,000 5. 6 8 2 1, 500 2. 5 2, 000 7. 3 8 2 1, 500 2. 5 2, 000 3. 6 8 2 1, 500 3. 0 2, 000 8. 6 9 l 1, 500 3. 0 2, 000 4. 2 9 1 1, 600 2.0 2,000 2. 7 9 1 1, 500 3. 5 2, 000 9. 2 9 1 l, 500 3. 5 2,000 7. 2 8 1 1 1,500 3. 5 2,000 0. 3 8 1 1 1, 500 3. 5 2,000 9. 1 8 1 1 l, 500 3. 5 2,000 8.7

As thus shown, a very desirable coating composition and method of coating is taught by addition of zirconium in quantities of 5 to 25% and silicon in quantities of 95 to 75% provided as an integral skin in combination with molybdenum upon a base metal. Such coating upon molybdenum is capable of imparting a long life to molybdenum when heated in air at extremely high temperatures, such as 2100 C., without fusing. Further additions of boron to the zirconium-silicon coating upon the molybdenum, such as 50 to silicon, '25 to 5% zirconium and 25 to 5% boron, provide a smoother non-blistering coating.

Our invention is not to be limited to the in situ forms of molybdenum-silicon-zirconium, with or without boron, skin on a molybdenum base, since the said skin may be preformed and applied either to a molybdenum base or a base of another metal or alloy, such as steel, nickel, titanium, etc., which is to be protected from high temperature oxidation, such as by having a skin of molybdenum first applied thereto as by cladding or electroplating the same, or by other known means for obtaining a molybdenum coating upon a refractory metal, the said molybdenum coated metal then having a skin applied thereto according to the procedures taught herein.

We claim:

1. As an article of manufacture, a refractory metal body resistant to oxidation in air at elevated temperatures comprising a molybdenum base having an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon and zirconium, the silicon and zirconium being in weight proportions of 75 to parts of silicon to 25 to 5 parts of zirconium and sintered into the molybdenum surface as a layer integral therewith.

2. As an article of manufacture, a refractory metal body resistant to oxidation inair at elevated temperatures comprising a molybdenum base having an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon and zirconium, the s111con and zirconium being in weight proportions of 80 to 90 parts of silicon and 20 to 10 parts of zirconium and sintered into the molybdenum surface as a layer integral therewith.

3. As an article of manufacture, a refractory metal body resistant to oxidation in air at elevated temperaturns moor n amclybdenum e hav n n x or 1m?! tscmpq ed PI dQIBin IY o an alloy n lic composition of molybdenum, silicon, zirconium and boron, the silicon, zirconium and boron being in Weight proportions of 50 to 90 parts of silicon, 25 to parts of zirconium, and 25 to 5 parts of boron.

4. As an article of manufacture, a refractory metal body resistant to oxidation in air at elevated temperatures comprising a molybdenum base having an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon, zirconium and boron, the silicon, zirconium and boron being applied by contact of the molybdenum with a composition containing silicon, zirconium and boron in proportions of .60 to 80 parts silicon, 20 to parts of zirconium, and to 10 parts of boron.

'5. As an article of manufacture, a refractory metal resistant .to oxidation in air at elevated temperatures comprising a molybdenum base having an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon and zirconium in the approximate Weight proportions of 59% molybdenum, 37% silicon and 4% zirconium.

6. As an article of manufacture, a refractory metal resistant to oxidation in air at elevated temperatures comprising a molybdenum base having an exterior layer composed predominantly of an alloy or intermetallic composition of molybdenum, silicon, zirconium and boron in the approximate Weight proportions of 59% molybdenum, 33% silicon, 4% zirconium and 4% boron.

7. As an article of manufacture, a refractory metal body resistant tooxidation at elevated temperatures in air comprising a molybdenum coated base metal having an exterior layer on said molybdenum coating composed predominantly of an alloy or intermetallic composition of molybdenum, silicon and zirconium in the approximate weight proportions of 50 to 75% of molybdenum, the balance being silicon and zirconium in the approximate weight proportions of'75 to 95% of silicon and to 5 of zirconium.

8. As an article of manufacture, a refractory metal body resistant to oxidation at elevated temperatures in air comprising a molybdenum coated base metal having an exterior layer on said molybdenum coating composed predominantly of an alloy or intermetallic composition of molybedenum, silicon, zirconium and boron in approximate weight proportions of to of molyb denum, the balance being silicon, zirconium and boron in the approximate weight proportions of 50 to of silicon, 25 to 5% of zirconium and 25 to 5% of boron. I

9. As an article of manufacture, a refractory metal body resistant to oxidation at elevated temperatures in] j air comprising a molybdenum base having an exterior layer composed predominantly of an alloy or inter-' metallic composition of molybdenum, silicon and zfir-. conium in the approximate weight proportions of 50 to 75% of molybdenum, the balance being silicon and zirconium in the approximate weight proportions of 75 to of silicon and 25 to 5% of zirconium.

10. As an article of manufacture, a refractory metal body resistant to oxidation in air at elevated temperatures comprising a molybdenum base having an exterior layer composed predominantly of an alloy or inter-' metallic composition of molybdenum, silicon, zirconium and boron in the approximate weight proportions of 50 to 75% of molybdenum, the balance being silicon, zir-1 conium and boron in the approximate weight proper-f tions of 50 to 90% of silicon, 25 to 5% of zirconium and 25 to 5% of boron.

References Cited in the file of this patent UNITED STATES PATENTS Wainer Sept. 28,

Weber June 10, 1924 

1. AS AN ARTICLE OF MANUFACTURE, A REFRACTORY METAL BODY RESISTANT TO OXIDATION IN AIR AT ELEVATED TEMPERATURES COMPRISING A MOLYBDENUM BASE HAVING AN EXTERIOR LAYER COMPOSED PREDOMINANTLY OF AN ALLOY OR INTERMETALLIC COMPOSITION OF MOLYBDENUM, SILICON AND ZIRCONIUM, THE SILICON AND ZIRCONIUM BEING IN WEIGHT PROPORTIONS OF 75 TO 95 PARTS OF SILICON TO 25 TO 5 PARTS OF ZIRCONIUM AND SINTERED INTO THE MOLYBDENUM SURFACE AS A LAYER INTEGRAL THEREWITH. 